The present invention relates in general to wireless communication systems and in particular to systems and methods for estimating and correcting DC offset in the presence of carrier frequency offset.
Wireless personal communication devices have proliferated over the past several years. Integration of more functionality such as multimedia capabilities into these devices has created an increasing demand for enhanced broadband communication methodologies. In addition, the promise of convenient, portable access to e-mail and the World Wide Web has fueled demand for inexpensive, compact, low-power devices.
Wireless devices generally communicate using radio-frequency (RF) technologies, in which a carrier RF wave is modulated by a data signal having a signal frequency distribution. RF receivers are designed to receive the RF-modulated signal and extract the data signal for further processing. In digital communication systems, this further processing is generally done after conversion of the extracted analog signal back to the digital domain. In standard heterodyne receivers, the data signal is extracted by mixing the received RF signal with the output of a first local oscillator operating at a frequency less than the carrier frequency, thereby generating an intermediate-frequency (IF) signal. The IF signal is then filtered and amplified before being converted to the baseband. Conversion to the baseband generally involves mixing the IF signal with the output of a second local oscillator operating at the intermediate frequency.
Recently, there has been increased interest in direct-conversion, or zero-IF, receivers as an alternative to heterodyne architectures. In zero-IF receivers, there is one local oscillator operating at the carrier frequency, and the received signal is converted directly to the baseband without IF signal processing. Such receivers typically require simpler analog components than heterodyne receivers (e.g., analog filters and amplifiers for zero-IF receivers operate in the baseband rather than at a nonzero intermediate frequency) and consume less power. Because zero-IF receivers can operate at lower power and be more easily integrated into monolithic systems than heterodyne receivers, such receivers are recognized as potentially very useful for applications where low cost, low power consumption, and small size are important, such as various wireless mobile handheld devices.
Zero-IF receivers, however, are susceptible to noise from sources including DC offset and carrier frequency offset. “DC offset” refers to a nonzero voltage that appears at the mixer output in the absence of a data signal. DC offset is caused, for instance, by current leakage from the receiver's local oscillator (which operates at the carrier frequency) to the mixer or other RF components, e.g., an RF amplifier. This leakage current can be propagated into the mixer, leading to a DC offset in the baseband signal.
Carrier frequency offset (CFO) arises from the finite tolerance of RF components used in the transmitter and receiver. Even though the carrier frequency of the transmitter is usually known to the receiver, due to RF tolerance, the frequency at which the receiver operates may not match the transmitter frequency exactly. Additionally, in systems where the transmitter or receiver is mobile, Doppler effects may also give rise to carrier frequency offsets. This offset causes a time-dependent phase shift in the received signal after conversion to the baseband that can cause errors in reconstruction of the transmitted signal.
In the absence of CFO, DC offset can be measured and corrected in the digital domain using a training sequence included in the transmitted signal. For instance, the IEEE 802.11a standard for wireless communication provides for long and short training sequences to be included in the transmitted data. These training sequences are made up of symbols that do not include any DC component; in the absence of DC offset, the average of samples over a period of a training symbol would be zero. Thus, averaging received samples corresponding to symbols of either training sequence can be used to estimate DC offset. But CFO introduces a time-dependent phase shift that causes the average of samples over the training sequence period to be non-zero even in the absence of DC offset, rendering this technique unreliable.
It would therefore be desirable to provide a more reliable technique for estimating and correcting a DC offset in the digital domain in the presence of CFO.